uld be said that some of the fixed stars are not isolated suns
like our own, but are composed of two great spheres revolving about
one another; hence they are termed double stars. The motions of these
bodies are very peculiar, and their conditions show us that it is not
well to suppose that the solar system in which we dwell is the only
type of order which prevails in the celestial families; there may,
indeed, be other variations as yet undetected. Still, these
differences throw no doubt on the essential truth of the theory as to
the process of development of the celestial systems. Though there is
much room for debate as to the details of the work there, the general
truth of the theory is accepted by nearly all the students of the
problem.

A peculiar advantage of the nebular hypothesis is that it serves to
account for the energy which appears as light and heat in the sun and
the fixed stars, as well as that which still abides in the mass of our
earth, and doubtless also in the other large planets. When the matter
of which these spheres were composed was disseminated through the
realms of space, it is supposed to have had no positive temperature,
and to have been dark, realizing the conception which appears in the
first chapter of Genesis, "without form, and void." With each stage of
the falling in toward the solar centres what is called the "energy of
position" of this original matter became converted into light and
heat. To understand how this took place, the reader should consider
certain simple yet noble generalizations of physics. We readily
recognise the fact that when a hammer falls often on an anvil it heats
itself and the metal on which it strikes. Those who have been able to
observe the descent of meteoric stones from the heavens have remarked
that when they came to the earth they were, on their surfaces at
least, exceedingly hot. Any one may observe shining meteors now and
then flashing in the sky. These are known commonly to be very small
bits of matter, probably not larger than grains of sand, which,
rushing into our atmosphere, are so heated by the friction which they
encounter that they burn to a gas or vapour before they attain the
earth. As we know that these particles come from the starry spaces,
where the temperature is somewhere near 500 deg. below 0 deg. Fahr., it is
evident that the light and heat are not brought with them into the
atmosphere; it can only be explained by the fact that when they ent